Airborne geophysical measurements

ABSTRACT

This invention concerns a method of making airborne geophysical measurements. Such measurements may be made from fixed or moving wing airplanes or dirlgibles. The method comprises the following steps: taking first real time measurements from one, or more, geophysical instruments mounted in an aircraft to produce geophysical data related to the ground below that instrument. Taking second real time measurements from navigation and mapping instruments associated with or carried by the aircraft. Computing a background response of each geophysical instrument using the second real time measurements to take account of its time varying altitude, and the time varying topography of the ground below it. Adjusting an operating or data processing condition of each geophysical instrument using the respective background response and the intrument&#39;s altitude to enhance the performance of that instrument. And, adjusting the geophysical data output for that instrument having reduced effects resulting from variations in altitude, attitude and topography.

TECHNICAL FIELD

This invention concerns a method of making airborne geophysicalmeasurements. Such measurements may be made from fixed or moving wingairplanes or dirigibles.

BACKGROUND ART

Airborne geophysical measurements have been made using aircraft equippedwith a geophysical instrument At different times, different geophysicalinstruments have been used, such as:

A magnetometer has been used to measure the distortions and additions tothe magnetic field of the earth due to the rocks and minerals below theaircraft.

An electromagnetic (EM) sounding system has been used to measure theeffects of the electrical conductivities of the rocks and minerals belowthe aircraft.

A radiometric survey system has been used to measure the radioactiveemanations from the radioactive isotopes of the elements that are theconstituent components of the rocks and earth below the aircraft.

A gravimeter sensor, and more recently a gravity gradiometer has beenused to measure the gravitational field, from which the density of therocks and minerals below the aircraft can be inferred.

A hyperspectral scanner has been used to measure the reflectance spectraof the rocks, earth and vegetation below the aircraft.

The interpretation of geophysical data collected from airbornemeasurements using such pieces of equipment takes place on the ground ina geological office. The purpose of the interpretation is to establishpriorities for subsequent investigation on the ground. Frequently, datais combined from several types of measurements obtained from differentspecialist geophysical aircraft, and at different times, to assist inthe interpretation.

SUMMARY OF THE INVENTION

In a first aspect, the invention is a method of making airbornegeophysical measurements, comprising the following steps:

Taking first measurements as a function of time from one, or more,geophysical instruments associated with or carried by at least oneaircraft to produce geophysical data related to the ground below thatinstrument.

Taking second measurements as a function of time from navigation andmapping instruments associated with or carried by the at least oneaircraft.

Computing a background response of each geophysical instrument as afunction of time using the second measurements to take account of itstime varying altitude, and the time varying topography of the groundbelow that instrument.

Adjusting data processing conditions applied to the geophysical datafrom each geophysical instrument using the respective backgroundresponse and the instruments attitude to enhance the performance of thatinstrument And,

Adjusting the geophysical data using the respective background A,response to yield a geophysical data output for that instrument havingreduced effects resulting from variations in altitude, attitude andtopography.

The first and second measurements may be taken in real time.

The first and second measurements may be recorded on a recording mediumto allow future retrieval of the measurements.

The step of computing a background response may take place, in realtime, within the aircraft during flight or after the flight iscompleted.

Using the resulting geophysical data output, or several outputs fromdifferent geophysical instruments, it may be possible to identifyexploration targets, and to compute their size and other key parameters,such as density, electrical conductivity and magnetic properties.

The second measurements may be used to compute:

The trajectory of the aircraft and the individual geophysicalinstruments in three dimensional space as a function of time.

The attitude (pitch, roll and yaw) of the individual geophysicalinstruments as a function of time. And,

A three-dimensional mathematical model of the ground below the aircraftas a function of time, and may be created instantaneously.

And the background response may be computed from these measurements whenthe data is analysed, and may be performed in real time.

Time varying adjustment and/or data processing conditions may becalculated for each geophysical instrument from the background responseand each instrument's attitude.

The geophysical instruments include one or more of a one or moremagnetic surveying instruments, an electromagnetic (EM) sounding system,a radiometric survey system, a gravimeter sensor, a gravity gradiometer,and a hyperspectral scanner.

A scalar magnetometer is used to measure the magnitude of the ismagnetic vector, a vector magnetometer to measure three orthogonalcomponent of the magnetic vector. A magnetic gradiometer is used tomeasure the six independent terms of the magnetic tensor.

The electromagnetic sounding system is used to measure the effects ofthe electrical conductivities of the rocks and minerals below theaircraft.

A radiometric survey system is used to measure the radioactiveemanations from the radioactive isotopes of the elements that are theconstituent components of the rocks and earth below the aircraft

The gravity gradiometer system is used to measure the gradient of theearth's gravitational field, and may also yield the attitude of theaircraft in three dimensional space, and the vertical velocity andacceleration of the aircraft A gravimeter is used to measure themagnitude of the earth's gravity.

A hyperspectral scanner is used to measure the reflectance of the earth,rocks and vegetation below the aircraft; and a radar altimeter todetermine the altitude.

More than one geophysical instrument may be used, and they may bemounted in the same or different aircraft, which may be the same or adifferent aircraft from the one in which the navigation and mappinginstruments are mounted.

In this case where geophysical data has been acquired by more than onegeophysical instrument mounted in the same aircraft, the method may beenhanced by using the measurements taken from each instrument toidentify and remove correlated errors in the measurements. Such errorsmay include residual height, topography, and attitude errors inmagnetic, gravity, radiometrics, hyperspectral and electromagneticinstruments.

The navigation and mapping instruments may include an inertialnavigation system, a GPS or DGPS and a topographic measuring system,such as a scanner. An optical (laser) scanning system or microwavescanning system may be used, for example, a Synthetic Aperture Radar(SAR). A radar or other altimeter may also be used.

The inertial navigation system may be used to determine the scanningsystem's position and orientation.

The GPS may be used to determine the position of the scanning system.

A scanning system may be used to emit pulses which reflect off theterrain below the aircraft. The scanning system may be able to measurethe topography of the terrain to an accuracy of 1 meter over a distanceof 1 to 2 times the aircraft's height on either side of the track of theairborne platform.

Other ancillary equipment may also be included, such as a data loggingsystem.

The geophysical sensor may be mounted in the same aircraft as thenavigation and mapping instruments. Alternatively, the geophysicalsensors may be mounted in separate but related aircraft, such as a birdwhich is towed from behind the aircraft in which the navigation andmapping instruments are mounted.

The GPS and data from the inertial navigation system may be processedtogether to derive the best detailed trajectory of the aircraft and theindividual geophysical instruments as a function of time, and possiblyin real time. This trajectory may be integrated with the scanning rangesto provide the three-dimensional mathematical model of the terrainsurveyed by the aircraft at each instant of time.

The mathematical model may be made up of a 3-dimensional array of volumeelements. The array may extend above and below the ground, and over adistance of up to twice the aircraft's height on either side of thepoint immediately below the aircraft at a particular time. Each volumeelement makes at least an inverse square contribution to eachinstruments reading.

The three-dimensional mathematical model may be used to compute themagnetic, electrical, radiometric, gravity and hyperspectral backgroundresponses of each geophysical sensor due to the variations in thetrajectory, and the topography, and may be computed in real time.

The computed background response and attitude for each instrument areused to make continuous time varying adjustments to the data processingconditions being applied to the data from the instrument and toeliminate the background response from the data outputs. The computedbackground response may be instantaneous.

Adjustments to the data processing conditions may include using thebackground response as a differential reference signal for theInstrument itself, in order to significantly reduce the dynamic range.Alternatively, filter characteristics, or other processing parametersmay be adjusted to obtain an optimum signal to noise ratio in theprocessed data.

Using the invention, it may be possible to map the geophysicalproperties of the area being surveyed with greater accuracy. Further itmay be possible to estimate the size of exploration targets with greateraccuracy, and improve the accuracy of the estimates of their geophysicalproperties. This could increase the probability of success in subsequentexploration.

Utilisation of several forms of airborne geophysical measurementtogether may improve the accuracy of each of the several measurementtechniques. It may jointly reduce the errors inherent in each type ofmeasurement as a consequence of the properties of the airborneenvironment It may jointly provide estimates of the several physicalproperties of the ground surveyed by the aircraft that are diagnostic ofthe nature and characteristics of the rocks and minerals therein. It mayallow the reduction of the survey duration in locations where access orweather factors impose severe operational restrictions. It may simplifyanalysis of the several geophysical measurements. It may reduce thecosts and operational overheads of the geophysical survey.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to theaccompanying drawing, in which:

FIG. 1 is a diagram of an aircraft equipped for geophysical surveys; and

FIG. 2 is a flow diagram of control inputs to geophysical instrumentsmounted in the aircraft of FIG. 1.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring first to FIG. 1, the aircraft 10 is equipped for makingairborne geophysical measurements. The equipment comprises a range ofgeophysical instruments indicated generally at 11, and a range ofnavigation and mapping instruments, indicated generally at 12.

Among the geophysical instruments 11 are:

A magnetic sensor or sensors 20 are included to measure the distortionsis and additions to the magnetic field of the earth due to the rocks andminerals below the aircraft The sensors may be one or more of thefollowing (a) a scalar magnetometer that measures the magnitude of themagnetic vector; (b) a vector magnetometer that measures threeorthogonal components of the magnetic vector; and (c) a magneticgradiometer that measures the six independent terms of the magnetictensor. One or more of these sensors may be mounted in the “bird” 23being towed behind the aircraft 10.

An electromagnetic (EM) sounding system 21 to measure the effects of theelectrical conductivities of the rocks and minerals below the aircraftThe EM system includes a transmitter to send out the electromagneticsignal and a receiver to sense the electromagnetic echo. The receiver 22may be installed in a “bird” 23 that is towed behind the aircraft 10.

A radiometric survey system 24 is also included to measure theradioactive emanations from the radioactive isotopes of the elementsthat are the constituent components of the rocks and earth below theaircraft 10.

A gravity gradiometer system 25 is used to measure the gradient of theearth's gravitational field. The gradiometer system 25 may include ahigh performance inertial navigation unit 30, providing the attitude ofthe aircraft, and the vertical velocity and acceleration of the airborneplatform. A gravimeter 26 may also be provided.

A hyperspectral scanner 27 is also used.

Among the navigation and mapping instruments 12 are:

The high performance inertial navigation unit 30 that may be includedwithin the gradiometer system 25.

A GPS system 31, or differential GPS, mounted in the aircraft todetermine the position of the aircraft in space.

The data from all the geophysical and navigational instruments arerecorded as a function of time for processing, amalgamation, andinterpretation.

A topographical scanning system 32 is mounted in the aircraft. This maycomprise an optical scanning system to map the topography of the terrainto a distance of one to two times the aircraft height on either side ofthe track of the aircraft. Alternatively a microwave scanning system isable to see through vegetation, and thereby map the topography of theground surface below the vegetative canopy to a distance of one to twotimes the aircraft height on either side of the track of the aircraft10.

If A radar altimeter 33 is also mounted in the aircraft 10.

Other ancillary equipment may also be included, such as a data loggingsystem 41 and associated memory 42.

A geophysical survey is conducted, using the specialised instruments, byflying over the terrain of interest at a low altitude of 100 m orthereabouts. It is conventional practice to fly in a series of nominallyparallel survey lines until the total region to be surveyed has beencovered.

The gravity gradiometer system 25 is operable to respond to thevariations in density of the rocks and minerals in the vicinity of thepoint below the aircraft and thereby provides a key diagnosticexploration capability to airborne geophysics.

The magnetic, EM, radiometric and gravitational properties of a volumeof rock or minerals are controlled by the values of the magneticpermeability (μ) and remanence, the electrical conductivity (σ), massdensity (ρ), and the concentrations of the radioactive materialrespectively within the volume. For an isolated 3-dimensional rock unit,which is frequently the target in mineral exploration, they also dependupon the horizontal cross section of the target and its depth and depthextent. Given adequately error free measurements of the magnetic, EM,radiometric and gravity properties of a target of exploration interestgood estimates can be made of the volume, depth, and of the physicalproperties μ, σ, and ρ. These, together with the concentrations of theradioactive elements allow the mineralogical nature of the geophysicstarget to be estimated.

The inertial navigation unit 30 that is a component of the gravitygradiometer system 25 provides accurate measurements of the verticalvelocity and acceleration of the airborne platform ten or more times persecond. Using these data together with the GPS 31 and radar altimeter 33date, the detailed trajectory that was flown by the airborne platformmay be determined to within +/−0.5 m. The altitude of the aircraft abovethe terrain will still vary by greater than +/−20 m due to turbulence,topography, and pilot input

The scanners 32 measures the topography of the ground between theseveral flight pats used to map the gravitational characteristics of theground.

Accurate measurements of the trajectory together with the topographymeasurements made by the associated optical and optional microwavescanners 32 allows the data from geophysical instruments (magnetics, EM,radiometrics hyperspectral and gravity) to be corrected for height andtopography using the algorithm appropriate to each form of measurement.

In addition, the inertial navigation unit 30 that is a component of thegradiometer system 25 measures the attitude of the aircraft withaccuracy. As a result the combination of GPS 31, the inertial navigationsystem 30 and the scanning system 32 provides a superior measurement ofthe Instantaneous height of the aircraft compared to that obtained usinga radar altimeter 33. It also provides a high accuracy measurement ofthe vertical velocity and acceleration of the airborne platform.

The several measurements of magnetism, conductivity, gravity andradiometric properties all vary with airborne platform height accordingto an inverse square law or stronger to the vicinity of an inverse sixthpower. For a flying altitude of 100 m, the residual errors due toaltitude alone after correction-as-outlined above are those due to the+/−0.5 m error in the trajectory and the +/−1 m due to topography. Theresidual errors In the measurements are therefore 2-6% compared to thegeophysical signal from the ground for a noise free trajectory. Sincethe errors for all four physical measurements vary by inverse power lawsor stronger, the relative error between the several measurements is lessthan about 2%.

The optical and optional microwave scanners 32 associated with thegravity gradiometer system 25 provide measurements of the topographybetween the flight lines of the survey that are accurate to +/−1 m.These are used to construct a mathematical model of the terrain belowthe aircraft. Each volume element of this topography relative to thepoint on the ground that is immediately below the aircraft makes aninverse square, or stronger contribution to each of the magnetic, EM,gravity and radiometric signals at the aircraft 10 or the bird 23. Thetopographical model of the terrain derived from the scanning systemsallows these contributions to be removed from all five forms ofgeophysical measurement, in real time, after the completion of theflight, or during re-processing at a later time.

The attitude (ie— pitch, roll and yaw) of the airborne platformintroduces errors into EM sounding and radiometrics that may be as highas 10% and which are uncorrelated between different surveys. Theinertial navigation 14) is system 30 that may be a component of thegravity gradiometer system 25 provides an accurate measurement of theattitude of the airborne platform in excess of ten measurements persecond, and this permits compensation for these sources of absolute andrelative error. These corrections are not made in current practice.

In summary, the multi sensor platform described above and the dataprocessing applied will yield magnetic, EM, gravity, hyperspectral andradiometric data as a function of time with residual errors that are <6%in total, and substantially less relative errors. Such a set of dataprovides the ability to assess the nature of an exploration target inquantitative detail.

FIG. 2 displays a flow chart 50 describing the steps involved to enhancethe instantaneous performance of each instrument Navigational sensors 12represent the inertial navigation system 30, GPS 31, and an opticaland/or microwave, scanner 32, respectively. The output of sensors 30 and31 are combined to calculate the trajectory path at 51. The output ofscanner 32 is combined with the trajectory path 51 to produce a relativeearth model at 52. The background response for each geophysicalinstrument in the system is generated from the relative earth model 52.

Geophysical instruments 11 include the magnetic sensors 20, a gravitysystem 25, an EM system 21 and a radiometric system 24. The backgroundresponse for each geophysical instrument is combined with the respectiveattitude measurement 49 (from the inertial navigation system 30) at 53to generate an adjustment parameter 55, 56, 57 and 58 for eachinstrument. The adjustment parameters are a function of time and each isused to vary the operating controls of respective instruments to enhancethe performance of the instrument.

The adjustment parameter 55, 56, 57 and 58 for each instrument are alsoused to adjust the outputs of each instrument to produce correctedoutputs 60, 61, 62 and 63.

For the purposes of illustration, we consider an idealised threedimensional geophysical target of scale size L, of magnetic permeabilityμ, of electrical conductivity σ, and mass density ρ. We consider asituation where the compensation techniques outlined above have greatlyreduced the errors in the geophysical data due to variable altitude, andtopography. We indicate these residual second order terms (that iserrors of order 1%) by the symbol O(2).

The magnetic anomaly from the target is proportional to μL³+O(2).

The EM anomaly at “early” times (about 1 millisec) is the so calledresistive limit response and varies as σL²+O(2). The EM anomaly at“late” times (10-20 millisec) varies as σL³+O(2).

The gravity gradient or gravity anomaly varies as ρL²+O(2).

In some cases the radiometric signal varies as the product of L²+O(2),and depends as well upon the concentration of the radioactive isotopesnear the earth's surface that have been produced by the weathering ofthe target mineralisation or host material.

Note in particular that the error terms are small compared to the signalfrom the target itself as a result of the accurate correction for heightand topography as detailed above.

Thus there are four or more simultaneous equations involving the fourdominant unknowns L, μ, σ, and ρ. As a consequence the equations can besolved for the four variables L, μ, σ, and ρ. The errors due to theminor unknowns of order 1% will introduce errors of no more than 10%into each of the unknowns L, μ, σ, and ρ. Together these parametersindicate the likely size of the target, and the physical parameters μ,σ, and ρ constrain the possibilities for the nature of themineralisation in the target Errors of 10% in any of them areinsignificant at this stage of exploration.

By way of comparison, consider the situation in the case where thegeophysical measurements are made by a number of different Instrumentson the same or different aircraft at different times, and without theability to correct for altitude, topography, and attitude variations.There will be uncorrelated errors of between +/−40% and +/−80% in eachof the magnetic, gravity, and radiometric measurements, and +/−80% inthe EM measurements due to the errors in the altitude, topography andattitude. Then the equivalent proportionalities to those given aboveinvolve 2N more independent unknowns, representing the altitude,attitude and topography errors, that are of a magnitude comparable tothe geophysical signals themselves, where N is the number of differentflights used to assemble the data sets. This set of simultaneousequations is not soluble for the unknowns L, μ, σ, and ρ. Independentestimates of some of these parameters can be made on the basis ofassumed values for one or more of others of them, however the errors inthe resulting estimates are then the Combination of the residual errorsin the data (40-80%) and those due to the assumptions made which may behundreds of percent in error. Thus the estimates of L, μ, σ, and ρ willbe usually in error by hundreds of percent.

In contrast, using the invention could result in a reduction of theerrors in the parameters L, μ, σ, and ρ from hundreds of percent to theorder of 10%.

1. A method of making airborne geophysical measurements, comprising thefollowing steps: taking first measurements as a function of time, fromone or more geophysical instruments associated with or carried by atleast one aircraft, to produce geophysical data related to the groundbelow that instrument; taking second measurements as a function of timefrom navigation and mapping instruments associated with or carried bythe at least one aircraft; computing a background response of eachgeophysical instrument as a function of time using the secondmeasurements to take account of its time varying altitude, and the timevarying topography of the ground below that instrument; adjusting dataprocessing conditions applied to the geophysical data from eachgeophysical instrument using the respective background response and theinstrument's attitude to enhance the performance of that instrument;and, adjusting the geophysical data using the respective backgroundresponse to yield a geophysical data output for that instrument havingreduced effects resulting from variations in altitude, attitude andtopography.
 2. A method of making airborne geophysical measurementsaccording to claim 1, where the first and second measurements are takenin real time.
 3. A method of making airborne geophysical measurementsaccording to claim 1, where the first and second measurements arerecorded on a recording medium to allow future retrieval of themeasurements.
 4. A method of making airborne geophysical measurementsaccording to claim 1, 2 or 3, where the geophysical data output is usedto identify exploration targets, and to compute their size and other keyparameters, including density, electrical conductivity and magneticproperties.
 5. A method of making airborne geophysical measurementsaccording to claim 1, where the second measurements is used to computeany one or more of: the trajectory of the aircraft and the individualgeophysical instruments in three dimensional space as a function oftime; the attitude (pitch, roll and yaw) of the individual geophysicalinstruments as a function of time; and, a three-dimensional mathematicalmodel of the ground below the aircraft as a function of time and thebackground response is computed from these measurements when the data isanalysed.
 6. A method of making airborne geophysical measurementsaccording to claim 1, where time varying adjustment or data processingconditions are calculated for each geophysical instrument from thebackground response and each instrument's attitude.
 7. A method ofmaking airborne geophysical measurements according to claim 1, where thegeophysical instruments include a one or more magnetic surveyinginstrument, such as a scalar magnetometer to measure the magnitude ofthe magnetic vector, a vector magnetometer to measure three orthogonalcomponent of the magnetic vector, and a magnetic gradiometer to measurethe six independent terms of the magnetic tensor.
 8. A method of makingairborne geophysical measurements according to claim 1, where thegeophysical instruments include an electromagnetic (EM) sounding system,to measure the effects of the electrical conductivities of the rocks andminerals below the aircraft.
 9. A method of making airborne geophysicalmeasurements according to claim 1, where the geophysical instrumentsinclude a radiometric survey system to measure the radioactiveemanations from the radioactive isotopes of the elements that are theconstituent components of the rocks and earth below the aircraft.
 10. Amethod of making airborne geophysical measurements according to claim 1,where the geophysical instruments include a gravimeter sensor, tomeasure the magnitude of the earth's gravity.
 11. A method of makingairborne geophysical measurements according to claim 1 where thegeophysical instruments include, a gravity gradiometer to measure thegradient of the earth's gravitational field.
 12. A method of makingairborne geophysical measurements according to claim 11, where thegravity gradiometer also yields the attitude of the aircraft in threedimensional space, and the vertical velocity and acceleration of theaircraft.
 13. A method of making airborne geophysical measurementsaccording to claim 1, where the geophysical instruments include ahyperspectral scanner to measure the reflectance of the earth, rocks andvegetation below the aircraft.
 14. A method of making airbornegeophysical measurements according to claim 1, where the geophysicalinstruments include a radar altimeter to determine the altitude.
 15. Amethod of making airborne geophysical measurements according to claim 1,where the geophysical instruments are mounted in the same or differentaircraft, which is the same or a different aircraft from the one inwhich the navigation and mapping instruments are mounted.
 16. A methodof making airborne geophysical measurements according to claim 15, wherethe geophysical instruments are mounted in different aircraft, and oneis towed behind the other in which the navigation and mappinginstruments are mounted.
 17. A method of making airborne geophysicalmeasurements according to claim 15, where the geophysical data has beenacquired by more than one geophysical instrument mounted in the sameaircraft, the measurements taken from each instrument is used toidentify and remove correlated errors in the measurements, includingresidual height, topography, and attitude errors in magnetic, gravity,radiometrics, hyperspectral and electromagnetic instruments errors. 18.A method of making airborne geophysical measurements according to claim1, where the navigation and mapping instruments includes a topographicmeasuring system, such as a scanning system to emit pulses which reflectoff the terrain below the aircraft to measure the topography of theterrain to an accuracy of 1 meter over a distance of one to two timesthe aircraft's height on either side of the track of the airborneplatform.
 19. A method of making airborne geophysical measurementsaccording to claim 18, where the scanning system is an optical (laser)scanning system or microwave scanning system, such as a SyntheticAperture Radar (SAR).
 20. A method of making airborne geophysicalmeasurements according to claim 18 or 19, where the navigation andmapping instruments includes an inertial navigation system, to determinethe scanning system's position and orientation.
 21. A method of makingairborne geophysical measurements according to claim 18, or 19, wherethe navigation and mapping instruments includes a GPS or DGPS, todetermine the position of the scanning system.
 22. A method of makingairborne geophysical measurements according to claim 1, where thenavigation and mapping instruments includes a radar or other altimeter.23. A method of making airborne geophysical measurements according toclaim 1, where the navigation and mapping instruments includes otherancillary equipment, such as a data logging system.
 24. A method ofmaking airborne geophysical measurements according to claim 20 or 21,where the GPS arid data from the inertial navigation system is processedtogether to derive the trajectory of the aircraft and the individualgeophysical instruments as a function of time.
 25. A method of makingairborne geophysical measurements according to claim 24, where thetrajectory may be integrated with the scanning ranges to provide thethree-dimensional mathematical model of the terrain surveyed by theaircraft at each instant of time.
 26. A method of making airbornegeophysical measurements according to claim 25, where the mathematicalmodel is made up of a 3-dimensional array of volume elements, the arraymay extend above and below the ground, and over a distance of up totwice the aircraft's height on either side of the point immediatelybelow the aircraft at a particular time, and each volume element makesat least an inverse square contribution to each instrument's reading.27. A method of making airborne geophysical measurements according toclaim 25 or 26, where the three-dimensional mathematical model is usedto compute the magnetic, electrical, radiometric, gravity andhyperspectral background responses of each geophysical instrument due tothe variations in the trajectory, and the topography.
 28. A method ofmaking airborne geophysical measurements according to claim 1, where thebackground response is computed instantaneously and attitude for eachinstrument is used to make continuous time varying adjustments to thedata processing conditions being applied to the data from thegeophysical instrument and to eliminate the background response born thedata outputs.
 29. A method of making airborne geophysical measurementsaccording to claim 1, where adjustments to the data processingconditions include using the background response as a differentialreference signal for the instrument itself, in order to significantlyreduce the dynamic range.
 30. A method of making airborne geophysicalmeasurements according to claim 1, where filter characteristics, orother processing parameters may be adjusted to obtain an optimum signalto noise ratio in the processed data.